thinkpad-acpi: forbid the use of HBRV on Lenovo ThinkPads

Forcing thinkpad-acpi to do EC-based brightness control (HBRV) on a
X61 has very... interesting effects, instead of doing nothing (since
it doesn't have EC-based backlight control), it causes "weirdness" in
the fan tachometer readings, for example.

This means the EC register that used to be HBRV has been reused by
Lenovo for something else, but they didn't remove it from the DSDT.

Make sure the documentation reflects this data, and forbid the user
from forcing the driver to access HBRV on Lenovo ThinkPads.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>

1 files changed, 8 insertions, 6 deletions

diff --git a/Documentation/laptops/thinkpad-acpi.txt b/Documentation/laptops/thinkpad-acpi.txt
index 88fc0661de56..f2ff638cce8d 100644
--- a/Documentation/laptops/thinkpad-acpi.txt
+++ b/Documentation/laptops/thinkpad-acpi.txt
@@ -1169,17 +1169,19 @@ may not be distinct.  Later Lenovo models that implement the ACPI
 display backlight brightness control methods have 16 levels, ranging
 from 0 to 15.
 
-There are two interfaces to the firmware for direct brightness control,
-EC and UCMS (or CMOS).  To select which one should be used, use the
-brightness_mode module parameter: brightness_mode=1 selects EC mode,
-brightness_mode=2 selects UCMS mode, brightness_mode=3 selects EC
-mode with NVRAM backing (so that brightness changes are remembered
-across shutdown/reboot).
+For IBM ThinkPads, there are two interfaces to the firmware for direct
+brightness control, EC and UCMS (or CMOS).  To select which one should be
+used, use the brightness_mode module parameter: brightness_mode=1 selects
+EC mode, brightness_mode=2 selects UCMS mode, brightness_mode=3 selects EC
+mode with NVRAM backing (so that brightness changes are remembered across
+shutdown/reboot).
 
 The driver tries to select which interface to use from a table of
 defaults for each ThinkPad model.  If it makes a wrong choice, please
 report this as a bug, so that we can fix it.
 
+Lenovo ThinkPads only support brightness_mode=2 (UCMS).
+
 When display backlight brightness controls are available through the
 standard ACPI interface, it is best to use it instead of this direct
 ThinkPad-specific interface.  The driver will disable its native